A shift control unit of an automatic transmission is disclosed in JP-A-6-42622. The shift control unit proposed in this document functions to switch an engaging hydraulic pressure of a releasing side frictional engaging element from a slow drain state to a quick drain state at the time of up-shift attained by releasing one frictional engaging element and engaging another frictional engaging element based on the output of a shift signal. It also includes an engine torque detector for detecting engine torque, a timing time deciding device for deciding a timing time from the output of the shift signal to the start of the quick drain state non-stepwise when the shift signal is outputted based on the detected signal from the engine torque detector, and an operation controller for changing from the slow drain state during the timing time decided by the timing time deciding device to the quick drain state after the elapse of the timing time. This arrangement is intended to prevent an engine blow and a shift shock to allow the driver to obtain the optimum shift feeling by switching from the slow drain state to the quick drain state at the optimum change-over timing corresponding to the engine torque.
Because the slow drain state is switched to the quick drain-state at the optimum switching timing corresponding to the engine torque in the control unit described above, the state cannot be changed from the slow drain state to the quick drain state at the optimum switching timing when the engine torque itself changes or varies.
The engine torque obtained on the basis of the throttle opening angle and engine speed changes or varies to a not negligible degree due to individual variation in engine performance and its elapsed change, to the ON/OFF condition of the air-conditioner and to changes in use conditions such as those in running up/down a slope especially when the output of the engine is low as is well known in general. Accordingly, situations arise with the shift control unit described above in that it is unable to reliably prevent engine blow and shift shock when the change or the variation of the engine torque is great.
In light of the foregoing, a need exists for a shift control unit that is able to improve the shift feeling so that the slow drain state is changed to the quick drain state at the optimum switching timing even in a range in which the engine torque changes or varies to a not negligible degree.